1. Field of the Invention
Example embodiments generally relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ an endless fixing belt having a heat capacity smaller than that of a fixing roller to shorten a warm-up time required to warm up the fixing belt to a given fixing temperature at which the toner image is fixed on the recording medium and thereby save energy. FIG. 1 illustrates a fixing device 90 incorporating an endless fixing belt 91. As shown in FIG. 1, a pressurization assembly 97 presses a pressing roller 92 against a nip formation pad 93 via the fixing belt 91 to form a fixing nip N between the pressing roller 92 and the fixing belt 91. Inside a loop formed by the fixing belt 91 are a reinforcement 94 that supports the nip formation pad 93, a heat conductor 96 disposed opposite the inner circumferential surface of the fixing belt 91, and a heater 95 disposed opposite the fixing belt 91 via the heat conductor 96. A temperature sensor 98 is disposed opposite the outer circumferential surface of the fixing belt 91 to detect the temperature of the fixing belt 91.
As the fixing device 90 is powered on, the heater 95 heats the heat conductor 96 which in turn heats the fixing belt 91 and at the same time the pressing roller 92 rotates clockwise in FIG. 1, which in turn rotates the fixing belt 91 counterclockwise in FIG. 1. As a recording medium P bearing a toner image T is conveyed through the fixing nip N formed between the pressing roller 92 and the fixing belt 91, the fixing belt 91 and the pressing roller 92 apply heat and pressure to the recording medium P, melting and fixing the toner image T on the recording medium P.
As the pressing roller 92 rotates the fixing belt 91, it stretches the fixing belt 91 toward the fixing nip N, bringing an upstream portion of the fixing belt 91 disposed upstream from the fixing nip N in the direction of rotation of the fixing belt 91 into contact with the heat conductor 96 and thereby facilitating heat conduction from the heat conductor 96 to the fixing belt 91. Conversely, a downstream portion of the fixing belt 91 disposed downstream from the fixing nip N in the direction of rotation of the fixing belt 91 is isolated from the heat conductor 96, decreasing heat conduction from the heat conductor 96 to the fixing belt 91. That is, a downstream portion of the heat conductor 96 disposed opposite the downstream portion of the fixing belt 91 is unnecessarily heated by the heater 95. Accordingly, the heat conductor 96 spanning substantially the entire inner circumferential surface of the fixing belt 91 decreases heating efficiency for heating the fixing belt 91, obstructing shortening of the warm-up time of the fixing belt 91 and saving energy.
To address this problem, the downstream portion of the heat conductor 96 disposed opposite the downstream portion of the fixing belt 91 may have a decreased thickness or a plurality of through-holes that decreases heat capacity of the downstream portion of the heat conductor 96, thus shortening the warm-up time of the fixing belt 91 and saving energy. However, even with the decreased thickness or the through-holes, the downstream portion of the heat conductor 96 may still draw heat from the heater 95. Additionally, producing the heat conductor 96 having different thicknesses and the through-holes may increase manufacturing costs.
Alternatively, a heat conduction roller having a diameter smaller than that of the heat conductor 96 and therefore having a decreased heat capacity may partially contact the inner circumferential surface of the fixing belt 91. As a heater disposed inside the heat conduction roller heats the heat conduction roller, the heat conduction roller heats the fixing belt 91. However, heat may dissipate from the heat conduction roller at a portion thereof isolated from the fixing belt 91, decreasing heating efficiency for heating the fixing belt 91.